Just wrote a long response but it seems to have gone AWOL. I hope you enjoy the new car, it sounds like a good deal.
I think the ECU has a running in mode, as the change at around 3500 miles was so dramatic. Economy is still below reasonable expectation but, its clear the two modes have wildly different fuel maps and its not like just shifting a curve on a graph up or down when using the eco button, more like two totally different cars with totally different characters. Once you accept this its easier to understand that the same throttle position in normal clearly uses hugely different fuel delivery in the different modes. I suggest learn the car in normal mode and find the sweet spots and only then start using eco mode or it will drive you mad how you never seem to be able to get decent economy. The system is totally bonkers but if you understand this I think in the end you will master the thing. At east it goes quite well if you just cane it, but most of the time I want a compromise involving economy. Anyway enjoy!!